Electronic image sensors are found in a variety of useful products, including cameras, camcorders, cell phones, medical devices, machine vision instruments, and the like. Image sensors have a characteristic dynamic range. Typical image sensors sense light by converting impinging photons into electrons that are integrated (collected) in sensor pixels. After completion of an integration cycle, the collected charge is converted into a voltage and supplied to an output terminal of the image sensor.
For example in complementary metal-oxide-semiconductor (CMOS) image sensors the pixels themselves perform the charge-to-voltage conversion to form an analog pixel voltage. The image sensor transfers the analog pixel voltage to the output terminal using various pixel addressing and scanning schemes. The image sensor can also convert the analog signal to a digital equivalent on-chip before reaching the chip output. Typical CMOS sensor pixels incorporate a buffer amplifier, typically a source follower (SF), which drives the sense lines that are connected to the pixels when selected by suitable addressing transistors. After the charge-to-voltage conversion is completed and the resulting signal transferred out from the pixels, the pixels are reset in order to be ready for accumulation of new charge packets. In a pixel that uses a floating diffusion (FD) as a charge detection node, the reset is accomplished by turning on a reset transistor that conductively connects the FD node to a voltage reference, which is typically the pixel drain node. The reset step removes collected charge, but in the process generates noise known as kTC-reset noise, as is well known in the art. Known CMOS image sensors remove the kTC-reset noise by using the correlated double sampling (CDS) signal processing technique in order to achieve the desired low noise performance. Typical CMOS image sensors that utilize the CDS technique usually require three or four transistors (4T) within each pixel. An example of the 4T pixel circuit with the pinned photodiode can be found in the U.S. Pat. No. 5,625,210 to Lee.
In the previous art the pixel circuits that consist of components such as the source follower transistor, the reset transistor, and the addressing transistor were all placed on the same substrate with the photodiodes. Also all the signal processing circuits, including the CDS and ADC circuits, were placed on the same substrate. This degree of integration presented some limitations and barriers for further sensor performance improvements. Recently, however, a significant progress has been made that allows now a direct chip-to-chip bonding with small metal pad interconnects. The pad-to-pad bonding can be performed for each pixel, or a group of several pixels sharing the same bonding pad, of the image sensor array. This concept is called the Hybrid Chip Stacking Technology (HCS) and this technology development now allows placing the light sensing photodiodes on the one chip and the remaining pixel circuits on another chip that is attached below the chip that is sensing light.
This disclosure, therefore, describes the pixels of photodiode arrays that are suitable for the BSI applications, which can be attached using the HCS technology, to a carrier chip that includes the pixel like circuit cells and the remaining signal processing circuitry. The chip-to-chip connections are performed at the Floating Diffusion (FD) node level, which presents some challenges related to the increased node parasitic bonding pad capacitance, the pixel to pixel uniformity and kTC reset noise. Possible solutions to these challenges are the topic of this patent disclosure.
The use of the same reference symbols in different drawings indicates similar or identical items. Unless otherwise noted, the word “coupled” and its associated verb forms include both direct connection and indirect electrical connection by means known in the art, and unless otherwise noted any description of direct connection implies alternate embodiments using suitable forms of indirect electrical connection as well.